Proper ID key to defect control.

At the height of the Cold War, a reporter dressed himself in the uniform of a Red Army officer. He walked around Washington D.C., rode the shuttle train to New York, strolled Wall Street and Times Square, used an exaggerated Russian accent to ask (and received) information from police officers and pedestrians for directions to various sensitive locations and freely entered public buildings. He was never challenged.

The reporter gave up his story idea about being caught as a spy, and, instead, wrote one about people "looking but not seeing."

Most of the time, he concluded, people see what they expect or are used to seeing. During the Cold War, people were not expecting to see an "enemy" officer in their midst, so they mentally mislabeled him as an Allied soldier. After all, what would a Russian officer be doing in America in 1953?

Cast metal defects in the foundry are like what that reporter experienced. Defects are easily recognizable if one takes the time to know their "uniforms," but, like the reporter, defects often masquerade as something they aren't. Defects can look similar but be caused by totally different problems.

This apparent family resemblance can lead the foundryman to snap identity conclusions, but be off the mark as to proper remedial steps. A misdiagnosed defect can lead to a wrong cure that could leave the true defect cause unchecked, or worse, add further defects.

Metalcasting is not a simple matter. It involves a wide array of variables, and the failure of any one can cause all manner of casting defects that are as old as metalcasting itself. Analyses of ancient castings made thousands of years ago have revealed various defects that would be readily recognized by modern metalcasters.

Nomenclature 

Casting quality requires limiting the damage caused by defects according to any given part, process, customer specification or requirement. Not every part or customer's need are alike. A disqualifying defect for one part or customer could be an acceptable level for another.

Correctly identifying a defect begins by correctly identifying the type of defect, properly classifying it as to cause and then applying those practical measures required for its elimination. All of this sounds easy enough, but correctly identifying defects is a major problem for metal casters.

Defects can look like one thing to one observer and another thing to a second. One "reads" a void as specifically being caused by gas entrained in the molten metal during molding. Another inspector, however, looking at the same part, interprets the cause as slag inclusions, an entirely different genesis.

Standardization

Several decades ago, the International Committee of Foundry Technical Associations addressed the need to classify casting defects and subsequently published the International Atlas of Casting Defects.

The book categorizes defects into a short master list each followed by a descending "tree" of subordinate, but unique, characteristics directly related to a stipulated defect cause.

AFS had the work translated and offers it as a textbook to clarify the difficult, often ambiguous, effort to identify and remediate casting defects. It remains a definitive work in the delineation of damaging defects, and indicates the probable causes and corrective steps required for each.

While not wholly accepted as gospel by all foundrymen, this widely used system of defect identification offers a unique approach to the problem. It lists seven basic categories of defects. The list was established by general consent among many international cast metal experts. Each of the seven is classified by a letter:

* A--metallic projections;

* B--cavities;

* C--discontinuities;

* D--defective surfaces

* E--incomplete casting;

* F--incorrect dimensions or shape;

* G--inclusions or structural abnormalities.

Each of these broad categories is divided into groups and subgroups designated by numerals. Completing the classification is a short definition of the defect, followed by the possible causes of the defect and finally offering design and/or metallurgical steps available to remediate the defect. Thus, B would refer to a cavity defect and a subgroup (for example, 221) would indicate internal or blind shrinkage; B 222, would also refer to a cavity but in this case, the 222 would indicate centerline or axial shrinkage.

Some defects can logically be classified under more than one category. Scabs would be an example. They are metallic projections (category A) yet they are classified as a subgroup (D230) under surface defects. Similarly, sand inclusions (G131) are also related by origin to several other defects. In general, this need cause no confusion in defect identification, particularly if diagnosis is made strictly on the basis of physical appearance without trying to first assign a name to the defect.

It would be unrealistic to list a single cause for each defect because casting imperfections more often result from a combination of circumstances rather than from a single well-defined cause. Nevertheless, the atlas lists causes for each that seem to be the most probable in terms of the current state of foundry technology.

The determination of the defect remedy results from an understanding of the causes. Much of the corrective action is the result of properly defining the defect in terms of the casting process itself and considering all of the supporting steps that go into the process of casting metal.

Correct information, the cost of process changes, customer requirements and the value of the parts being cast are among the factors to be considered in dealing with practical defects control.

Bibliography

International Atlas of Casting Defects, AFS Publications (1974).

Analysis of Casting Defects, 3rd Ed., AFS Publications (1974).

Casting Defects Handbook, AFS Publications (1972).